Liquid crystal display has the advantages such as large screen, high resolution, small and portable, low price and flat screen, etc. However, since the liquid crystal itself cannot emit light, the image has to be displayed relying on a backlight source or ambient light. Thus, the liquid crystal also has the shortcomings such as narrow visual angle, slow response (magnitude of millisecond), and not useful under low temperature, etc. Therefore, people attempt to find a new light emitting material that can replace liquid crystal materials, and thus OLED (Organic Light-Emitting Diodes) and PLED (Polymer Light-Emitting Diodes) have been produced. OLED/PLED has the advantages such as self-illuminating, wide visual angle, high contrast, low power consumption, fast response, full color and simple manufacture, etc.
With the OLED/PLED techniques gradually becoming mature, they have already been used in the filed of flat display, such as some terminal devices like computer, cell phone, and the like. Compared to traditional liquid crystal flat display (TFT-LCD), OLED/PLED displays have the advantages such as lighter and thinner, low power consumption, clear and bright color and wide visual angle.
Generally, OLED/PLED devices have a multilayer structure comprising a hole transport layer (HTL), an emitting layer that converts electric energy to luminous energy, an electron transport layer (ETL), etc. Its mechanism of operation is as follows: under the effect of an external electric field, a hole injected by an anode and an electron injected by a cathode, after passing through the hole transport layer and the electron transport layer, complex with each other in the emitting layer, and interact to produce an activated exciton which will generate an energy difference by radiative transition when it returns to a ground state from an excited state, and finally releases luminous energy in the form of photons. The hole transport layer and the electron transport layer serve to maintain a balance between the injections of the holes and the electrons.
Most of OLED/PLED devices have a sandwich structure. That is, an organic film having semi-conductor properties is sandwiched by electrodes on two sides, and at least one side is a transparent electrode, such as an ITO (indium tin oxide) layer, so as to obtain a planar illumination.
OLED device usually has several organic material layers between the metal cathode and the ITO anode. The device structure is usually formed by evaporation coating one layer by one layer with a vacuum evaporation technique. Evaporation coating has low material utilization, high requirements for devices and is not applicable for large scale production.
In contrast, PLED is manufactured by using a solution procedure, including spin coating and inject printing method. It has the advantages of high material utilization and applicability for manufacture of large scale and large size products. One difficulty of the solution procedure is that the upper layer solution may dissolve the organic molecules which have already formed into a film, thereby affecting the efficiency of the materials and the performance of the device. In order to protect the polymer film already formed from being dissolved into the solvent, the polymer film may be cross-linked to form a polymer network which would not be dissolved by the solvent.